Transistor

ABSTRACT

The invention relates to a transistor having an emitter ( 1 ), a collector ( 2 ), and a base layer ( 3 ), wherein the emitter ( 1 ) extends into the base layer ( 3 ), wherein the base layer ( 3 ) has an intrinsic region ( 4 ) arranged between the emitter ( 1 ) and the collector ( 2 ), and an extrinsic region ( 6 ) that runs between the intrinsic region ( 4 ) and a base contact ( 5 ), wherein the base layer ( 3 ) contains a first doping layer ( 7 ) doped with a trivalent doping substance, which extends into the extrinsic region ( 6 ) and which is counter-doped by means of a pentavalent counter-doping substance ( 8 ) in the region of the emitter ( 1 ). The electrical resistance of the base layer ( 3 ) can be reduced, in advantageous manner, by means of the first doping layer ( 7 ).

The invention relates to a transistor with an emitter, a collector, anda base layer.

Transistors of the type noted above are known from the publication “SiGeBipolar Technology for Mixed Digital and Analog RF Applications,” J.Bock et al., IEEE 2000, in which the base layer has an intrinsic segmentand an extrinsic segment, where the extrinsic segment connects a basecontact with the intrinsic segment. The extrinsic segment has arelatively low boron doping, so that the known transistor has thedisadvantage of a high resistance in the base layer. This leads to adrop in power amplification already at low frequencies, and thereby toan effective slow-down of the transistor. In addition, the higher basefeed resistance causes greater noise.

It is the goal of the present invention to provide a transistor wherethe base layer has a low ohmic resistance.

This goal is achieved by means of a transistor according to claim 1.Advantageous embodiments of the transistor are shown in the dependentclaims.

A transistor is disclosed that has an emitter, a collector, and a baselayer. The emitter extends into the base layer. The base layer has anintrinsic region arranged between the emitter and the collector.Furthermore, the emitter has an extrinsic region that runs between abase contact and the intrinsic region of the base layer. The base layercontains a first doping layer that runs within the base layer, which isdoped with a trivalent doping substance. The first doping layer extendsinto the extrinsic region and also runs in the region of the emitter,where it is counter-doped with a pentavalent doping substance.

The transistor has the advantage that doping of the base layer can beundertaken via the first doping layer, which extends into the extrinsicregion and also runs into the emitter, thereby advantageously reducingthe ohmic resistance of the base layer. In this way, electrical lossesof the transistor can be reduced.

Because the first doped layer runs both in the extrinsic region of thebase layer and in the region of the emitter, it can be produced viausual methods for doping base layers, without any additional structuringstep. Usual methods are: doping by means of implantation, as well asepitactic depositing.

Via the doping, additional charge carriers in the form of holes in thebase layer are made available, thereby increasing the conductivity ofthe base layer. In this way, the ohmic resistance between the basecontact, where the base layer is contacted from the outside, and theintrinsic base is reduced.

Advantageously, boron can be selected as a trivalent doping substancefor the first doping layer. Boron has the advantage that its activationenergy of the holes is the lowest of all trivalent doping substances. Asa result, doping with boron already works at room temperature.

Additional doping layers can be arranged between the first doping layerand the collector. A second doping layer and a third doping layer areenvisioned, for example. The second doping layer is arranged between thefirst doping layer and the third doping layer. The second and thirddoping layers are each doped with a trivalent doping substance,preferably boron. The doping-substance concentration of the seconddoping layer is less than the doping-substance concentration of thefirst and third doping layers.

The low doping-substance concentration of the second doping layer hasthe advantage that in this way, the PN transition on the base side comesto rest in a region with a low doping-substance concentration. In thisway, the emitter/base leakage current on the basis of the tunnel effectis reduced, on the one hand, and, on the other hand, the parasiticemitter/base capacitance is minimized.

The pentavalent doping within the base layer can be diffused into thebase layer from an emitter region that borders on the base layer. Thisdiffusion of a pentavalent doping substance from the emitter region intothe base layer is advantageous, since in this way, the PN transition canbe shifted from a polycrystalline silicon, usually used for the emitterregion, to a region of the base layer having crystalline silicon. Thishas the result that the PN transition lies in a region having fewinterference locations, and for this reason, the resulting transistorhas a better direct-voltage characteristic, with good linearity of theamplification.

In the following, the invention will be explained in greater detail,using an example embodiment and the related diagrams.

FIG. 1 shows a silicon substrate with a transistor, in schematiccross-section.

FIG. 2 shows the concentration progression of doping substances alongline A in FIG. 1.

FIG. 1 shows a silicon substrate having a base layer 3. An emitterregion 11 is arranged above the base layer 3. A collector is arrangedbelow the base layer 3. The base layer 3 has an intrinsic region 4 thatlies between the collector 2 and the emitter 1 of the transistor. Theemitter 1 is formed from the emitter region 11 and a region that has acounter-doping substance 8, which is diffused into the base layer 3 fromthe emitter region 11. The broken line in FIG. 1 shows the edge of thecounter-doping substance 8.

The base layer 3 furthermore comprises an extrinsic region 6 that runsbetween a base contact 5 and the intrinsic region 4.

Furthermore, a first doping layer 7, which runs within the extrinsicregion 6 and also within the emitter 1, is envisioned in the base layer3. The first doping layer 7 is preferably produced by doping with boron.Measured according to a depth scale that begins at the top end of thearrow A (at t0), the first doping layer 7 begins at a depth t1. Itextends to a depth t2. In the region between the emitter region 11 andthe collector 2, the first doping layer 7 lies completely within theemitter 1. A second doping layer 9 is connected to the first dopinglayer 7. The second doping layer 9 extends from depth t2 to depth t4.The second doping layer 9 has a lower doping than the first doping layer7. The second doping layer 9 is connected to a third doping layer 10.The third doping layer 10 extends from depth t4 to depth t5. Thecollector 2 then begins at depth t5. The third doping layer 10 has ahigher doping than the second doping layer 9. Preferably, all threedoping layers 7, 9, 10 are produced by the doping substance boron.

On the depth scale along the line A, the counter-doping substance 8extends to depth t3, which means that the counter-doping substance 8still extends into the second doping layer 9.

FIG. 2 shows the dependence of dopings on concentration along the line Ain FIG. 1. Here, the doping-substance concentration C is plotted as afunction of the depth t. C4max represents the maximal doping-substanceconcentration of the counter-doping substance 8 in the region of thebase layer 3. Depth to marks the boundary between the emitter region 11and the base layer 3. This is, at the same time, the boundary between asilicon material that is present in polycrystalline form (emitter region11) and in monocrystalline form (base layer 3). The first doping layer 7begins at a distance from this boundary layer between the emitter region11 and the base layer 3. The first doping layer 7 has a doping substanceconcentration C1 that is essentially constant over the layer thicknesst2−t1. The doping substance concentration C1 is preferably between1×10¹⁸ and 5×10²⁰ cm⁻³. The thickness t2−t1 of the first doping layer 7is preferably between 10 and 100 nm.

Directly connected to the first doping layer 7 is the second dopinglayer 9. The doping-substance concentration in the second doping layer 9is essentially constant and corresponds to the doping-substanceconcentration C2. C2 preferably lies between 1×10¹⁸ and 1×10¹⁹ cm⁻³. Thethickness t4−t2 of the second doping layer 9 is selected in such amanner that at least half of the second doping layer 9 still lies withinthe region delimited by the counter-doping substance 8 and the outerboundary of the region that represents the emitter 1. This isadvantageous for realizing a low parasitic emitter/base capacitance.

The third doping layer 10 still lies next to the second doping layer 9.The third doping layer 10 has a thickness t5−t4, which typically amountsto 5 to 50 nm. The doping substance concentration C3, which isessentially constant within the third doping layer 10, is preferablybetween 5×10¹⁸ and 1×10²⁰ cm⁻³.

In this regard, it is particularly advantageous if the doping substanceconcentration C1 of the first doping layer 7 has a sizable proportion ofthe total amount of doping substance in the base layer 3. In this way,it can be assured that the first doping layer 7 makes a significantcontribution to the conductivity of the base layer 3. It is advantageousif the proportion of the first doping layer 7 comprises 30% or more ofthe total amount of doping substance that is determined by the firstdoping layer 7, together with the second doping layer 9 and the thirddoping layer 10.

The reference symbols indicated in the lower part, below the abscissa,as well as in the upper part of FIG. 2, correspond to the referencesymbols used in FIG. 1 for the individual layers.

Furthermore, the counter-doping substance 8 can be seen in FIG. 2; itproceeds from a maximal doping-substance concentration C4max and atfirst remains constant with increasing depth, and then decreasesgreatly, approximately at the bottom edge of the first doping layer 7,and finally is reduced to zero within the second doping layer 9. Thecounter-doping substance 8 marks the outermost edge of the emitter 1. Ithas the effect that the first doping layer 7 present in the base layer3, which increases the ohmic resistance of the extrinsic base, does nothave any negative effects in the intrinsic part of the transistor. Here,the counter-doping is designed in such a way that the doping of thefirst doping layer 7 is at least compensated, and preferably is actuallyover-compensated.

Depending on the boundary between the emitter 1 and the intrinsic region4 of the base, the intrinsic region 4 extends approximately between thedepth t5 and the depth t3, while the emitter 1 extends between the deptht3 and the left edge of FIG. 2. The monocrystalline base layer delimitsthe emitter region vertically. Laterally, it is defined viaphotolithography, so that the diffusion of As is effective only in theintrinsic region.

In FIG. 2, a germanium doping 12 can also be seen, which decreases,proceeding from the collector 2, towards the base. By means of such aramp-shaped germanium doping 12, an acceleration of the charge carrierspenetrating into the base from the collector 2 can take place, and thisincreases the speed of the transistor.

The maximal doping substance concentration of the counter-dopingsubstance 8, C4max, is preferably in the range between 1×10²⁰ and 1×10²¹cm⁻³. Preferably, arsenic is used as the counter-doping substance 8.This arsenic is diffused into the base layer 3 from the emitter region11.

It is also advantageous to ensure, by including carbon atoms in the baselayer 3 in a concentration range greater than 1×10¹⁸ cm⁻³, that thediffusion of the trivalent doping substance, in particular the diffusionof boron, is effectively reduced. In this way, the result can beachieved that the width of the base layer 3 can be reduced, therebyresulting in a higher cut-off frequency. The inclusion of carbon atomscan take place, for example, via co-deposition of carbon during theepitactic growth of the base layer 3.

1. A transistor comprising: an emitter; a collector; a base layer having a base contact, the base layer comprising: an intrinsic region between the emitter and the collector; an extrinsic region between the intrinsic region and the base contact; and a first doping layer that is doped with a trivalent doping substance, that extends into the extrinsic region, and that is counter-doped with a pentavalent substance in a region adjacent to the emitter.
 2. The transistor of claim 1, wherein the trivalent substance comprises boron.
 3. The transistor of claim 1, wherein the base layer further comprises: a second doping layer that is doped with a trivalent substance, and that is between the first doping layer and the collector; and a third doping layer that is doped with a trivalent substance, and that is between the second doping layer and the collector; a concentration of trivalent substance in the second doping layer is less than a concentration of trivalent substance in the first doping layer, and the concentration of trivalent substance in the second doping layer is less than a concentration of trivalent substance in the third doping layer.
 4. The transistor of claim 1, wherein the first doping layer comprises at least 30% of a total amount of a doping substance in the base layer.
 5. The transistor of claim 1, wherein the base layer further comprises: a substance diffused into the base layer from a region that corresponds to the collector.
 6. The transistor of claim 1, wherein the base layer comprises carbon atoms having at a concentration greater than 1×10¹⁸ cm⁻³.
 7. The transistor of claim 3, wherein the first doping layer comprises at least 30% of a total amount of a doping substance in the base layer.
 8. The transistor of claim 3, wherein the base layer further comprises: a substance diffused into the base layer from a region that corresponds to the collector.
 9. The transistor of claim 3, wherein the base layer comprises carbon atoms having a concentration greater than 1×10¹⁸ cm⁻³.
 10. The transistor of claim 3, wherein the trivalent substance comprises boron.
 11. The transistor of claim 3, wherein the second doping layer and the third doping layer are doped with germanium;
 12. The transistor of claim 11, wherein: a concentration of germanium in the second doping layer and the third doping layer decreases from a high point at the collector to a low point in the second layer; and a decrease in the concentration of germanium from the high point to the low point is substantially constant.
 13. A transistor comprising: a base layer comprising: a first doping layer that is doped with a trivalent substance; a second doping layer adjacent to the first doping layer and having a lower concentration of the trivalent substance than the first doping layer; and a third doping layer adjacent to the second doping layer and having a higher concentration of the trivalent substance than the second doping layer; wherein the first doping layer and the second doping layer are counter-doped with a pentavalent substance in an emitter region of the transistor.
 14. The transistor of claim 13, wherein the second doping layer and the third doping layer are doped with germanium.
 15. The transistor of claim 14, wherein a concentration of germanium in the second doping layer and the third doping layer decreases from a high point at a collector region of the transmitter to a low point in the second layer.
 16. The transistor of claim 14, wherein a decrease in the concentration of germanium from the high point to the low point is substantially constant.
 17. The transistor of claim 11, wherein the trivalent substance comprises boron.
 18. The transistor of claim 11, wherein the pentavalent substance comprises arsenic having a concentration of between 1×10²⁰ cm⁻³ and 1×10²¹ cm⁻³.
 19. A transistor comprising: a collector region; an emitter region; and a base layer between the collector region and the emitter region, the base layer comprising: an intrinsic region between the collector and the emitter; and an extrinsic region outside the intrinsic region; wherein the intrinsic region and the extrinsic region comprise plural layers that are doped with different concentrations of a trivalent substance; and wherein at least some of the plural layers in the intrinsic region are doped, from the emitter region, with a pentavalent substance.
 20. The transistor of claim 19, wherein: at least some of the plural layers in the intrinsic region are doped, from the collector region, with germanium; a concentration of the germanium decreases from a high point at the collector region to a low point in one of the plural layers doped with the trivalent substance; and a decrease in the concentration of germanium from the high point to the low point is substantially linear. 